Among prior art apparatus of the type described above, one may refer to, for example, an apparatus as disclosed in U.S. Pat. No. 3,868,681 wherein an electromagnetic or electrostatic coupling between an electrical conductor net extending over an input board and an input pen is detected to determine the coordinate position designated by the input pen on the input board, or an apparatus as disclosed in Bremner IEEE Transaction C, 1970, June, PP 546-548 wherein an input pen which radiates an ultrasonic wave which is received by microphones disposed about an input board so as to detect the delay of the wave received by the microphones from the position designated by the input pen, thus determining the coordinate position designated by the input pen. However, they are not yet satisfactory from the standpoint of construction and performance.
More particularly, in the former, high accuracy is required for the electrical conductor net on the input board and the peripheral circuit also requires high degree of skill. In the latter, since the propagating velocity of the ultrasonic wave varies with ambient temperature and humidity, it is necessary to compensate for the variation in the velocity. Furthermore, in order to detect a small delay of the received wave, it is necessary to use complicated peripheral circuits, thus making it difficult to decrease the cost of manufacturing and to attain high reliability. Especially, in the latter, it is necessary to constantly supply to the input pen a signal necessary for radiating the ultrasonic wave through an electric cord. Thus, it has been impossible to drive the input pen by a self-contained battery, for example, which dispenses with the cord (cordless) for connection between the input pen and the apparatus.